Well, the narrator obviously forgot to mention about the hero that -during the war of the ring and the chase of Sauron's Ring- killed 2 dragons (one of them ressurected by a vicious wight and a Nazgul which you also killed), killed thousands of Orcs, helped Aragorn reforge his blade by killing a huge turtle in Evendim, killed the Witch King and a few of the other 9, killed dozens of giants, and humongous spiders (possibly spawns of Ungoliant), and helped hundreds of people by doing thousands of quests in Middle-Earth.
Tolkien failed to mention all those deeds (most among them being mighty). Therefore corroding the very core of your game. But not mine.
For some reason you won't accept this. Instead you will only go mad at Dwarves battling among Rohirrim at Helm's Deep (after of course they aided hundreds of them and becoming more known among them).
This is a fallacy. Oh no, you can't complain about {gameplay feature} unless you always complain about {all this other stuff} too! The fact that in the past, people (me included) have commented extensively on all that OTT stuff you mention seems to somehow get ignored. If we did talk about that as well, you'd probably be complaining about digging up old arguments instead. So no, I'm not going to play that sort of game. We're discussing one particular topic.
Besides which, mentioning all that OTT stuff reinforces the point that the typical player-character is portrayed as a super-hero and hence it's just not sensible for anyone to suggest that this paragon of valour is going to rock up to Helm's Deep, not do anything of note and not get noticed. The gameplay is bound to involve insanely overblown heroics just like it has before, since by now that's a well-established pattern.
Is it so far fetched for somebody that helped a whole country to be fighting among those people in their time of need?
No. Not at all. In fact it's most logical to do so. And to even be asked by the king to help.
Is anyone saying it's far-fetched in itself? I don't think so. All that's being said is that the ensuing super-heroics are very likely going to end up being lore-breaking since no such heroics actually went on or they'd have been noticed, talked about, spun into tales. It'd all be a change to the books - again, nothing wrong with that in itself, there's only this inexplicable need some people have to pretend nothing's being changed to accommodate it.